Catalytic composition based on transition metal complexes, and a process for the hydrogenation of unsaturated compounds

ABSTRACT

A novel catalytic composition comprises at least one quaternary ammonium and/or phosphonium salt in which the anion is selected from the group formed by tetrafluoroborate, tetrachloroborate, hexafluorophosphate, hexafluoroantimonate, hexafluoroarsenate, trifluorosulphonate, fluorosulphonate, tetrachloroaluminate, dichlorocuprate, and trichlorozincate, and at least one complex of a transition metal from groups 8, 9 and 10, i.e., iron, ruthenium, cobalt, rhodium iridium, nickel, palladium and platinum. Application to total or selective hydrogenation of unsaturated compounds such as monoolefins, diolefins, acetylenic compounds, aromatic compounds, polynuclear aromatic compounds. Hydrogenation may be accompanied by isomerisation.

FIELD OF THE INVENTION

An object of the present invention is to provide a novel catalytic composition and its use for hydrogenation and isomerisation of unsaturated compounds, the composition resulting from the interaction of an organic-inorganic salt which is liquid at the reaction temperature, hereinbelow termed a "molten salt", and a complex of a transition metal from groups 8, 9 and 10.

BACKGROUND OF THE INVENTION

A very large number of transition metal complexes are known which catalyse the hydrogenation of unsaturated compounds and which are soluble in suitable organic solvents or in the reactants and in the hydrogenation products. Those catalysts are dealt with in an article in "Principles and Applications of Organotransition Metal Chemistry", by J. P. Collman, University Science Books, Mill Valley, U.S.A.

United States patent U.S. Pat. No. 3,565,823 describes a composition consisting of a dispersion constituted by a compound, in particular of a transition metal in a salt of tin or germanium and a quaternary ammonium or phosphonium. U.S. Pat. No. 3,657,368 describes an olefin hydrogenation process and U.S. Pat. No. 3,919,271 describes a nitrile hydrogenation process, both using the above composition. U.S. Pat. No. 3,832,391 claims an olefin carbonylation process using the same composition.

The compositions described above have the disadvantage of having a relatively high melting point, the hydrogenation reaction thus taking place at 150° C., for example.

SUMMARY OF THE INVENTION

We have now discovered that complexes of transition metals from groups 8 and 9, in particular ruthenium, rhodium and iridium complexes, combined with an organic-inorganic salt which is liquid at low temperatures, can be used to hydrogenate unsaturated compounds.

One object of the invention is to provide a catalytic composition comprising at least one compound of a transition metal from groups 8, 9 and 10, in particular complexes of ruthenium, rhodium and iridium, and at least one quaternary ammonium and/or phosphonium salt, said composition resulting from at least partial dissolution of a transition metal compound in a "molten salt".

A further object of the invention is to provide a process for the hydrogenation of double or triple bonds between carbon atoms in unsaturated compounds, in which the unsaturated compound or compounds is/are brought into contact with a complex of a transition metal from groups 8 and 9, in particular ruthenium, rhodium and iridium, the compound being at least partially dissolved in a "molten salt". The "molten salt" medium is constituted by an organic cation and an inorganic anion. The hydrogenation products are slightly soluble or insoluble in the catalytic composition.

The "molten salts" of the invention are non aqueous, ionic in nature and have the general formula Q⁺ A⁻ where Q⁺ represents a quaternary ammonium and/or quaternary phosphonium cation and A⁻ represents any known anion which can form a liquid salt at low temperatures, i.e., below 150° C., advantageously at most 80° C., preferably below 50° C., such as the following ions: tetrafluoroborate, tetrachloroborate, hexafluorophosphate, hexafluoroantimonate, hexafluoroarsenate, trifluoromethylsulphonate, fluorosulphonate, tetrachloroaluminate, dichlorocuprate, and trichlorozincate. Anions selected from the group formed by trichlorocuprate, tetrachlorocuprate, heptachloroaluminate and decachloroaluminate can also be used. One particular catalytic composition (Ni) contains no aluminate ions. Halide and hydroxide ions are excluded from the invention. The quaternay ammonium and/or phosphonium ions preferably have the general formulae NR¹ R² R³ R⁴⁺ and P R¹ R² R³ R⁴⁺, or have general formulae R¹ R² N═CR³ R⁴⁺ and R¹ R² P═CR³ R⁴⁺, where R¹, R², R³ and R⁴, which may be identical or different, represent hydrogen with the exception of the cation NH₄₊, and preferably a single substituent can represent hydrogen, or hydrocarbon residues containing 1 to 12 carbon atoms, for example saturated or unsaturated, cycloalkyl or aromatic, aryl or aralkyl alkyl groups containing 1 to 12 carbon atoms. The ammonium and/or phosphonium ions can also be derived from nitrogen-containing or phosphorous-containing heterocycles containing 1, 2 or 3 nitrogen and/or phosphorous atoms, with the following general formulae: ##STR1## where the cycles are constituted by 4 to 10 atoms, preferably 5 to 6 atoms, R¹ and R² being as defined above. The quaternary ammonium or phosphonium cation can also be a cation with formula

    R.sup.1 R.sup.2 N.sup.+ ═CR.sup.3 --R.sup.5 --R.sup.3 C═N.sup.+ R.sup.1 R.sup.2

    R.sup.1 R.sup.3 P.sup.+ ═CR.sup.3 --R.sup.5 --R.sup.3 C═P.sup.+ R.sup.1 R.sup.2

where R¹, R² and R³, which may be identical or different, are as defined above and R⁵ represents an alkylene or phenylene residue. Examples of R¹, R², R³ and R⁴ groups are methyl, ethyl, propyl, isopropyl, butyl, secondary butyl, tertiary butyl, amyl, methylene, ethylidene, phenyl or benzyl radicals; R⁵ may be a methylene, ethylene, propylene or phenylene group. The ammonium and/or phosphonium cation is preferably selected from the group formed by N-butylpyridinium, N-ethylpyridinium, 3-butyl-1-methylimidazolium, diethylpyrazolium, 3-ethyl-1-methylimidazolium, pyridinium, trimethylphenylammonium, 3-ethyl-1-methylimidazolium, and tetrabutylphosphonium. Examples of suitable salts are N-butylpyridinium hexafluorophosphate, N-ethylpyridinium tetrafluoroborate, tetrabutylphosphonium tetrafluoroborate, 3-butyl-1-methylimidazolium hexafluoroantimonate, 3-butyl-1-methylimidazolium hexafluorophosphate, 3-butyl-1-methylimidazolium trifluoromethylsulphonate, pyridinium fluorosulphonate, trimethylphenylammonium hexafluorophosphate, 3-butyl-1-methylimidazolium tetrachloroaluminate, 3-butyl-1-methylimidazolium heptachloroaluminate, trimethylphenylammonium chloride, 3-ethyl-1-methylimidazolium chloride, tetrabutylphosphonium bromide, N-butylpyridinium chloride, N-ethylpyridinium bromide, 3-butyl-1-methylimidazolium chloride, diethylpyrazolium chloride, and pyridinium hydrochloride. These salts can be used alone or as a mixture. They act as a solvent. As a result, the solvent of the catalytic composition contains mainly this/these salt/salts. If other solvents are present, their essential function is to allow dissolution of the transition metal compound before mixing, it with the salt.

The transition metal compounds for use in accordance with the invention are in general all those transition metal complexes which are known to the skilled person. These are zero-, mono-, di- or trivalent compounds in which the metal is selected from the group formed by Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, advantageously from the group formed by Ru, Rh, Ir, Co, Pt, Pd, and preferably Ru, Rh, Ir, and can for example be bonded to halide, hexafluorophosphate, hexafluoroarsenate, tetrafluoroborate, tetrachloroborate, etc . . . ions, hydride ions, and hydrocarbon ligands such as cyclopentadienyls and substituted cyclopentadienyls, acetylacetonates and substituted acetylacetonates, or neutral ligands such as tertiary phosphines, ditertiary diphosphines, phosphites, olefins, carbon monoxide, or nitriles. These complexes of the invention may be mononuclear or polynuclear, neutral or ionic. They may contain a chiral ligand. Examples of complexes which can be used in the present invention are RuH⁴ (PPh₃)₂, RhCl(PPh₃)₃, Rh(norbornadiene) (PPh₃)₂ !⁺ PF₆ !⁻, IrCl(PPh₃)₃, HRh(CO) (PPh₃)₃, (C₅ H₅)RhCl₂, Rh(C₅ Me₅)Cl₂ (Ph₂ PCH₂ COPh), IrCl(CO)(PPh₃)₂, Rh(norbomadiene) (PPh₃)(Ph₂ PCH₂ COPh)!⁺ PF₆ !⁻, Rh(norbornadiene) (bisdiphenylphosphinoethane)!⁺ PF₆ !⁻, Rh(norbomadiene) (bisdiphenylarsinoethane)!⁺ PF₆ !⁻, Rh(norbomadiene) (DIOP)!⁺ PF₆ !⁻, Ru(norbornadiene)(C₅ Me₅ !⁺ BF₄ !⁻, (C₆ Me₆)RuCl₂ !₂, (C₆ H₆)RuCl₃ Ru(C₆ H₆)!PF₆, and RhCl(cyclooctene)₂ !.

These complexes can be prepared remote from the reaction medium and introduced into it for reaction. They can also be formed in situ in the reaction medium, by introducing the components necessary for their formation. Other transition metal compounds can be used, such as inorganic salts, oxides, hydroxides, or organic salts.

In general, the catalytic composition can contain an organic solvent such as aromatic hydrocarbon or a hydrocarbon-containing compound, but the quantity of organic solvent is lower than that of the molten salt. The catalytic composition does not contain water.

The concentration of the compound containing the transition metal complex in the molten salt is advantageously in the range 1 mmole of complex per liter of molten salt to 500 mmoles per liter, preferably in the range 2 mmoles per liter to 200 mmoles per liter, more preferably in the range 2 mmoles per liter to 100 mmoles per liter, most preferably in the range 2 mmoles per liter to 50 mmoles per liter.

The compounds in the composition of the invention can be mixed in any order at a temperature of more than -20° C., generally in the range -20° C. to at most 150° C., preferably in the range -20° C. to 140° C., advantageously in the range 0° C. to 120° C., most preferably 0°-80° C. or 0°-50° C.

A further object of the invention is to provide a catalyst based on a support impregnated with the catalytic composition described above.

The support generally has a specific surface area of 0.01 to 1500 m² /g, preferably 0.01 to 150 m² /g, or more preferably 0.01 to 50 m² /g. Its total pore volume is generally 0.005 to 1.5 cm³ /g, preferably 0.005 to 1 cm³ /g, more preferably 0.005 to 0.8 cm³ /g.

The support is selected from organic or inorganic supports, such as macroreticular resins, perfluorinated resins, charcoal, the oxides SiO₂, ZrO₂, TiO₂, SnO₂, Al₂ O₃, Fe₂ O₃, or HfO₂, or any combination of these oxides.

A process for preparing the catalyst comprises at least one calcining step for the support at a preferred temperature of 200°-600° C., followed by impregnation with the catalytic solution using any known method known to the skilled person.

A still further object of the invention is to provide a process for the hydrogenation of an unsaturated compound using a catalytic composition or a catalyst as defined above.

Unsaturated compounds which can be hydrogenated in accordance with the invention are simple olefins, diolefins (in particular conjugated diolefins), acetylenic compounds, and aromatic hydrocarbons (in particular polynuclear aromatic hydrocarbons). They may contain one or more heteroatoms, in particular unsaturated heteroatoms, such as a ketone function or a carboxylic function. Examples are the hydrogenation of butene to butane, pentene to pentane, 1,3-butadiene to butene, vinylacetylene to butadiene, isoprene to 2-methylbutene, 1,5-cyclooctadiene to cyclooctene, 1,5,9-cyclododecatriene to cyclododecene, and anthracene to tetrahydroanthracene. Hydrogenation can be total or selective, for example diolefins to monoolefins. Hydrogenation may be accompanied by isomerisation, for example 1-butene to 2-butene, or 1-pentene to 2-pentene.

In the hydrogenation process, the unsaturated compound may be used pure or diluted by saturated or unsaturated hydrocarbons such as those found in cuts from various hydrocarbon refining processes. An example is butanes and/or butenes with butadiene.

The hydrogenation temperature is in the range -10° C. to 200° C., advantageously less than 150° C., preferably in the range +10° C. to less than 150° C. The total pressure or partial pressure of hydrogen is between atmospheric pressure or a pressure below atmospheric pressure and 20 MPa, preferably between atmospheric pressure and 10 MPa.

Catalytic hydrogenation of unsaturated compounds can be carried out in a closed system, a batch system or in a continuous system using one or more reaction stages. At the reactor outlet, the organic phase containing the reaction products is advantageously separated by simple settling out of the polar catalytic phase containing the molten salt and the major portion of the catalyst. The polar phase which contains at least a portion of the catalyst is, at least partially returned to the reactor, the other portion being treated to eliminate the remaining catalyst.

Examples of unsaturated compounds to which the process can advantageously be applied are butenes, pentenes, butadiene, isoprene, piperylene, and 1,5-cyclooctadiene.

The following examples illustrate the invention without in any way limiting its scope.

EXAMPLE 1

4 ml of 3-butyl-1-methylimidazolium hexafluorophosphate, 43.2 mg (0.05 mmole) of Rh(norbornadiene)(PPh₃)₂ !⁺ PF₆ ⁻ complex dissolved in 2 ml of acetone, and 2 ml of 1-pentene were introduced into a 50 ml capacity double-walled glass reactor which had been purged of air and moisture and placed under one atmosphere of hydrogen. Two liquid phases were observed, the lower clear phase being brown in colour and the hydrocarbon phase, colourless. The hydrogen pressure was taken to 0.1 MPa and the temperature taken to 30° C., and stirring was started. After 2 hours, stirring was stopped and the mixture was allowed to settle; the supernatant hydrocarbon phase was extracted. More than 99% of the 1-pentene had been converted, with 35% of the products being pentane and 65% of the products being 2-pentenes. A new charge of 1-pentene was introduced; after 2 hours of reaction, more than 99% of the 1-pentene had been converted. The reaction was repeated 3 more times using the same salt containing the same rhodium complex with the same results with no apparent loss of rhodium.

EXAMPLE 2

The salt of the above example containing the rhodium complex was taken and the volatile organic compounds were eliminated by placing it under vacuum. 2 ml of 1-pentene was introduced in a hydrogen atmosphere and the reaction was carried out under the conditions given in the above example. The same results were obtained and the test was repeated three more times.

EXAMPLE 3

The method of Example 1 was followed, with the exception that no acetone was introduced. The polar phase was brown in colour and the hydrocarbon phase was colourless. 95% conversion was obtained and 56% of the products was pentane. The test was repeated three more times with the same results.

EXAMPLE 4

The salt of the previous example containing the rhodium complex was taken, from which the volatile organic compounds had been eliminated by placing it under vacuum. 5 ml of heptane and 2 ml of 1-pentene were introduced in a hydrogen atmosphere under the conditions of that Example. The organic phase was colourless. More than 99% conversion was obtained and 62% of the products was pentane.

EXAMPLE 5

The salt of the previous example containing the rhodium complex was taken, from which the volatile organic compounds had been eliminated by placing it under vacuum. 18.4 mmoles of 1,3-cyclohexadiene was introduced in a hydrogen atmosphere under the conditions of Example 1. 92.3% conversion of the cyclohexadiene was obtained and 90% of the products was cyclohexene.

EXAMPLE 6

The method of Example 3 was followed, with the exception that the 4 ml of 3-butyl-1-methylimidazolium hexafluorophosphate was replaced by 4 ml of butylmethylimidazolium hexafluoroantimonate. The polar phase was red-brown in colour and the organic phase was colourless. 96% conversion was obtained, pentane representing 83% of the products and 2-pentenes, 17%. The test was repeated three more times with the same molten salt and the same rhodium complex, with the same results.

EXAMPLE 7

The method of Example 1 was followed, with the exception that the Rh(norbornadiene)(PPh₃)₂ !⁺ PF₆ ⁻ complex was replaced by 0.05 mmole of (norbornadiene)Rh(Ph₂ PCH₂ COPh)(PPh₃)!⁺ PF₆ ⁻ ! complex. The lower phase was orangey in colour and the hydrocarbon phase was colourless. After 2 hours stirring, the medium was allowed to settle and the hydrocarbon phase was extracted. 98% of the 1-pentene had been converted. The operation was repeated after eliminating the volatile organic compounds by placing under vacuum, then a further time by introducing 5 ml of heptane. The results were identical to that of the initial test.

EXAMPLE 8

4 ml of 3-butyl-1-methylimidazolium hexafluoroantimonate, 0.05 mmole of (norbornadiene)Rh(Ph₂ PCH₂ COPh)(PPh₃)₂ !⁺ PF₆ ⁻ ! complex and 2 ml of 1-pentene were introduced into a 50 ml capacity double-walled glass reactor which had been purged of air and moisture and placed under one atmosphere of hydrogen. Two liquid phases were observed, the lower clear phase being brown in colour and the hydrocarbon phase, colourless. The hydrogen pressure was taken to 0.1 MPa and the temperature taken to 30° C., and stirring was started. After 2 hours, stirring was stopped and the mixture was allowed to settle; the supernatant hydrocarbon phase was extracted. 81% of the 1-pentene had been converted, with 27% of the products being pentane and 73% of the products being 2-pentenes. The reaction was repeated two more times using the same salt containing the same rhodium complex with the same results with no apparent loss of rhodium.

EXAMPLE 9

4 ml of the salt used in Example 5 containing the rhodium complex and from which the volatile organic compounds had been eliminated by placing under vacuum was introduced into a 50 ml capacity double-walled glass reactor which had been purged of air and moisture and placed under one atmosphere of hydrogen. 81.5 mg of anthracene dissolved in 5 ml of benzene was introduced in a hydrogen atmosphere. Two liquid phases were observed, the lower clear phase being orange-brown in colour and the hydrocarbon phase, colourless. The hydrogen pressure was taken to 0.5 MPa and the temperature taken to 60° C. and stirring was started. After 15 minutes stirring was stopped and the mixture was allowed to settle; the supernatant hydrocarbon phase was extracted. 74% of the anthracene had been converted, 70% being constituted by tetrahydroanthracene and the remainder by dihydroanthracene.

EXAMPLE 10

4 ml of 3-butyl-1-methylimidazolium hexafluoroantimonate and 0.05 mmole of Rh(norbornadiene)(PPh₃)₂ !⁺ PF₆ ⁻ complex, which mixture had been used to hydrogenate 1-pentene and from which the volatile organic compounds had been eliminated by placing under vacuum, were introduced into a 50 ml capacity double-walled glass reactor which had been purged of air and moisture and placed under one atmosphere of hydrogen. 81.5 mg of anthracene dissolved in 5 ml of benzene was introduced in a hydrogen atmosphere. Two liquid phases were observed, the lower clear phase being orange-brown in colour and the hydrocarbon phase, colourless. The hydrogen pressure was taken to 0.5 MPa and the temperature taken to 60° C., and stirring was started. After 21 hours, stirring was stopped and the mixture was allowed to settle; the supernatant hydrocarbon phase was extracted. 54% of the anthracene had been converted, 88% of which was constituted by tetrahydroanthracene and the remainder by dihydroanthracene. 

We claim:
 1. A process for the hydrogenation of double or triple bonds between carbon atoms in unsaturated hydrocarbon compounds in the presence of a catalytic composition comprising a polar non-aqueous solvent which is ionic in nature and liquid at a temperature below 150° C. mainly containing at least one molten quaternary ammonium and/or phosphonium salt with general formula Q⁺ A⁻, where Q⁺ represents a quaternary ammonium and/or phosphonium cation, and A⁻ represents an anion, said composition also comprising at least one compound of at least one transition metal from groups 8, 9 or 10 dissolved in said non-aqueous solvent, said non-aqeuous solvent optionally further comprising an organic solvent in an amount less than said molten salt.
 2. A process according to claim 1, in which the anion is selected from the group consisting of tetrafluoroborate, tetrachloroborate, hexafluorophosphate, hexafluoroantimonate, hexafluoroarsenate, dichlorocuprate, tetrachloroaluminate, triluoroarsenate, trifluoromethylsulphonate, and fluorosulphonate, trichlorozincate.
 3. A process according to claim 1, in which said anion is selected from the group consisting of heptachloroaluminate, decachloroaluminate, trichlorocuprate and tetrachlorocuprate.
 4. A process according to claim 2, in which the transition metal is selected from the group consisting of ruthenium, rhodium, iridium, cobalt, platinum and palladium.
 5. A process according to claim 1, in which the transition metal is nickel and the anion is selected from the group consisting of tetrafluoroborate, tetrachloroborate, hexafluorophosphate, hexafluoroantimonate, hexaflouroarsenate, dichlorocuprate, trifluoromethylsulphonate, fluorosulphonate, trichlorocuprate, and tetrachlorocuprate, trichlorozincate.
 6. A process according to claim 1, in which the quaternary ammonium and/or phosphonium cation is selected from the group consisting of cations with the general formula: ##STR2## where R¹, R², R³ and R⁴, which may be identical or different, represent hydrogen, with the exception of NH₄ +, and hydrocaibon residues containing 1 to 12 carbon atoms, and in which the cycles are constituted by 4 to 10 atoms.
 7. A process according to claim 1, which the quaternary ammonium and/or phosphonium cation has one of the following general formula:

    R.sup.1 R.sup.2 N.sup.+ ═CR.sup.3 --R.sup.5 --R.sup.3 C═N.sup.+ R.sup.1 R.sup.2

    R.sup.1 R.sup.2 P.sup.+ ═CR.sup.3 --R.sup.5 --R.sup.3 C═P.sup.+ R.sup.1 R.sup.2

where R¹, R² and R³ which may be identical or different, represent hydrogen or hydrocarbon residues containing 1 to 12 carbon atoms, and R⁵ represents an alkylene or phenylene residue.
 8. A process according to claim 1, in which the quaternary ammonium and/or phosphonium cation is selected from the group consisting butylpyridinium, N-ethylpyridinium, 3-butyl-1-methylimidazolium, diethylpyrazolium, 3-ethyl-1-methylimidazolium, pyridinium, trimethylphenylammonium, 3-ethyl-1-methylimidazolium, and tetrabutylphosphonium.
 9. A process according to claim 1, in which the quaternary ammonium and/or phosphonium salts are selected from the group consisting of N-butylpyridinium hexafluorophosphate, N-ethylpyridinium tetrafluoroborate, tetrabutylphosphonium tetrafluoroborate, 3-butyl-1-methylimidazolium hexafluoroantimonate, 3-butyl-1-methylimidazolium hexafluorophosphate, 3-butyl-1-methylimidazolium trifluoromethylsulphonate, pyridinium fluorosulphonate, trimethylphenylammonium hexafluorophosphate, 3-butyl-1-methylimidazolium tetrachloroaluminate, 3-butyl-1-methylimidazolium heptachloroaluminate, trimethylphenylammonium chloride, 3-ethyl-1-methylimidazolium chloride, N-ethylpyridinium bromide, 3-butyl-1-methylimidazolium chloride, diethylpyrazolium chloride, pyrdinium hydrochloride, tetrabutylphosphonium bromide, N-butylpyridinium chloride.
 10. A process according to claim 1, in which the transition metal compound is a transition metal complex.
 11. A process according to claim 1, in which the transition metal in the complex is bonded to ionic or neutral ligands.
 12. A process according to claim 1, in which the metal compounds from groups 8, 9 or 10 are chlorides, bromides or iodides.
 13. A process according to claim 10, in which the complex is selected from the group consisting of RuH₄ (PPh₃)₂, RhCl(PPh₃)₃, Rh(norbornadiene) (PPh₃)₂ !⁺ PF₆ !⁻, IrCl(PPh₃)₃, HRb(CO)(PPh₃)₃, (C₅ H₅)RhCl₂, IrCl(CO)(PPh₃)₂, Rh(norbornadiene) (PPh₃)(Ph₂ PCH₂ COPh)!⁺ PF₆ !⁻, Rh(norbornadiene) (bisdiphenyphosphinoethane)!⁺ PF₆ !⁻, Rh(norbornadiene) (bisdiphenylarsinoethane)!⁺ PF₆ !⁻, Ru(norbornadiene)(C₅ Me₅ !⁺ BF₄ !⁻, (C₆ Me₆) RuCl₂ !₂, (C₆ H₆)RuCl₃ Ru(C₆ H₆)!PF₆.
 14. A process according to claim 1, in which the concentration of the complex of transition metals from groups 8 to 10 is 1to 500 mmoles per liter with respect to the ammonium and/or phosphonium salt.
 15. A process according to claim 1, in which the catalytic composition has been impregnated into a porous organic or inorganic support.
 16. A process according to claim 1, in which the unsaturated compound is a monoolefin.
 17. A process according to claim 1, in which the unsaturated compound is a diolefin or an acetylenic compound.
 18. A process according to claim 17, in which the diolefin or acetylenic compound is selectively hydrogenated to a monoolefin.
 19. A process according to claim 1, in which the unsaturated compound is a polynuclear aromatic compound.
 20. A process according to claim 19, in which the polynuclear aromatic compound is anthracene.
 21. A process according to claim 1, in which hydrogenation is accompanied by isomerisation.
 22. A process according to claim 1, in which the hydrocarbon phase is separated from the polar phase, said polar phase containing at least a major portion of the catalyst being recycled at least in part to the hydrogenation reactor.
 23. A process according to claim 1, carried out at a temperature between -10° C. and 200° C., at a pressure in the range 0.1 MPa to 20 MPa.
 24. A process according to claim 1, wherein the catalytic composition comprises 3-butyl-1-methylimiydazolium hexafluoroantimonate and Rh(norbornadiene)(PPh₃)₂ !⁺ PF₆ ⁻.
 25. A process according to claim 1, wherein said compound of at least one transition metal is present in the non-aqueous solvent in a range of 1 mmole to 500 mmole per liter of said molten salt.
 26. A process according to claim 1, wherein said compound of at least one transition metal is present in the non-aqueous solvent in a range of 2 mmole to 200 mmole per liter of said molten salt.
 27. A process according to claim 1, wherein said compound of at least one transition metal is present in the non-aqueous solvent in a range of 2 mmole to 50 mmole per liter of said molten salt.
 28. A process according to claim 1, wherein said organic solvent is an aromatic hydrocarbon or a hydrocarbon-containing compound.
 29. A process according to claim 1, wherein said non-aqueous solvent is essentially free of said organic solvent.
 30. A process according to claim 1, wherein said non-aqueous solvent contains said organic solvent.
 31. A process according to claim 1, wherein said catalytic composition is an unsupported catalyst. 